The invention relates to a method for the production of an electrotechnical device. In particular, the invention relates to the production of so-called MID devices (xe2x80x9cMoulded Interconnect Devicesxe2x80x9d) which are formed from a plastic moulding which intrinsically combines the functions of a conventional printed circuit board and mechanical fixing elements used to retain electronic components.
It is known in the prior art to produce such MID devices proceeding from plastic mouldings to which whole-area copper foil is applied by means of a hot stamping method. In this method, on the one hand the copper foil is structured so as to produce conductor tracks and on the other hand these conductor tracks are connected to the plastic moulding by the underlying plastic being melted on. This method makes it possible to produce only relatively coarse conductor structures which only have conductor tracks with similar widths.
It is also known to connect a structured copper foil to a plastic moulding e.g. by hot rolling. This method necessitates the difficult handling of copper foils which, mechanically, are extremely sensitive.
Furthermore, it is known for plastic films which are provided with electrical conductor tracks on one side or on both sides to be backed by injection-moulding with the same or a different plastic, as a result of which a 3-dimensional circuit support is produced. However, this method is suitable only for plastics which combine by melting in the course of the backing by injection-moulding, and cannot be used e.g. for liquid crystal polymers (LCPs).
Finally, it is known for conductor structures made of electrically conductive material such as conductive adhesive, for example, to be applied to a support sheet and for this to be transferred to a circuit support by means of an adhesive system, it being necessary to apply a separating means between the support sheet and the conductor tracks. Both the adhesive system and the separating means can adversely affect the properties of the arrangement in respect of their flammability.
Taking this as a departure point, the object of the invention is to specify a method according to which MID devices can be produced reliably, even with fine conductor track structures, and cost-effectively by virtue of the capability of industrial-scale manufacture.
This object is achieved by means of a method according to claim 1.
The method according to the invention has the advantage that the sensitive conductor track structures are formed on a support sheet which is mechanically and thermally insensitive and is simple to handle. This sheet can be supplied e.g. from a roll during the production of the MID device. The support sheet is peeled away only when the conductor track structure is fixedly connected to the plastic body of the subsequent MID device, with the result that damage to the conductor track during the production of the device is Practically precluded.
The conductor track structures can be produced by the electrodeposition of copper. For this purpose, firstly a whole-area, thin first copper layer is coated onto the support sheet. In order to obtain adhesion with a defined peel-off force between the support sheet and the first copper layer, an adhesion promoting layer may be coated onto the support sheet, which adhesion promoting layer may additionally contain palladium nuclei for implementing the metallization. The copper layer is subsequently built up to the desired thickness by electrodeposition in accordance with a mask which leaves the conductor routes free. Depending on the quality of the support sheet, it may also be necessary to clean the support sheet before the adhesion promoting layer is coated on. The economical consumption of copper in this method has an advantageous effect both directly during the production of the device and during the work-up of residues.
It is particularly advantageous if the support sheet chosen is a temperature-resistant, polyimide-based sheet, because this sheet does not adhere to the plastic body forming the circuit support. This makes it possible for the support sheet to be peeled away without additional separating means. Furthermore, the adhesion promoting layer which facilitates the metallization may aid,the reliable peeling-away of the support sheet from the conductor tracks when the latter are connected to the circuit support.
Since the first copper layer short-circuits the conductor track structures together, it may be necessary in many cases for this first copper layer to be removed again during the production process of the MID device. This can be done e.g. after the support sheet has been peeled away or before the plastic body is injection-moulded onto the conductor track sheet.
A soldering resist mask can be coated onto the structured module by means of a screen printing method. Furthermore, it is advantageous if the bare conductor tracks are passivated e.g. with rosin, in order to preserve the solderability of the conductor tracks.
In a particularly advantageous embodiment of the invention, the second electrically conductive layer, which forms the conductor tracks, is applied in such a way that the surface of the conductor tracks has numerous projections and depressions. When the plastic body is injection-moulded onto these conductor tracks, the projections act like anchors in the plastic, resulting in a very firm positively locking connection between the copper conductor tracks and the plastic body. This property is particularly advantageous if the plastic used is LCP plastic material (xe2x80x9cLiquid Crystal Polymerxe2x80x9d), which adheres poorly to smooth copper surfaces.
Furthermore, the invention relates to an electrotechnical device, in particular an MID device, having a base body made of plastic and also having electrical conductor tracks fixedly connected to the base body. Such devices intrinsically combine both electrical and mechanical functions and, in this way, enable e.g. a more space-saving design of electrical apparatuses. MID devices are composed to a considerable extent of plastic which, for safety reasons, is provided with a flameproofing agent. Furthermore, such devices contain metals. The mixture of different materials makes it more difficult to recycle such devices at the end of their service life.
Taking this as a departure point, a second object of the invention is to provide an electrotechnical device, in particular an MID device, which can be produced cost-effectively and has improved properties compared with conventional devices of this type.
This object is achieved by means of a device according to claim 18, which is characterized in that the conductor tracks have a multiplicity of depressions and projections which are suitable for imparting a positively locking connection to the base body made of plastic.
One advantage of this device is that the particular structuring of the conductor tracks means that it is now possible also to use plastics which adhere poorly to materials from which conductor tracks are usually produced. This applies e.g. to LCP plastic material, which is distinguished by its intrinsically flame-retarding properties. The use of this material has the advantage that it is possible to dispense with the additional use of flame-retarding substances, which frequently make it difficult to recycle used apparatuses.